Improving Accuracy and Efficiency of Aerodynamic Simulations for Heavy Vehicles
Typ
Examensarbete för masterexamen
Program
Engineering mathematics and computational science (MPENM), MSc
Publicerad
2020
Författare
Johansson, Simon
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Heavy-duty vehicles are a necessity in the current society but they answer to 6% of
the total CO2 emissions in Europe. Therefore new regulations are set by the European
Union to limit those emissions. These limits make the losses of the vehicles
an important factor and especially aerodynamic drag. Reducing the aerodynamic
drag calls for good developing methods and accurate predictions. This means that
validations and limitations of the methods are needed.
This thesis will investigate a CFD method by creating a base case simulation and
validate this against wind tunnel test. The method will use unsteady simulations to
capture transient behaviour in the flow. A statistical confidence intervals for the averaged
values as well as a starting point for the averaging will be presented. Further,
the spatial mesh will be studied and the effects of the number of inner iterations
will be presented. The validation of the base case will be stated as differences in
the coefficients Cd , Cs , and Cp to understand advantages and disadvantages of
the method. The frequency content from the time-resolved coefficients will also be
studied. When a well-defined base setup has been validated the temporal resolution
will be investigated to see how it affects the results. This will be done for increased
time steps as well as a significantly reduced time step. The significantly reduced
time step ensures the CFL number to be less than unity in the entire domain.
The validation of the base case shows that there is an error in Cd of 45 ± 7 drag
counts for the base case and that the error in pressure on the rear end of the truck
is yaw angle dependent. The error in Cd is almost fully explained by the error in
pressure on the rear end of the truck. The result from the simulations with different
time steps show that for a fully resolved flow very fine time steps are needed to
keep the CFL number less than unity in the majority of the domain, but if global
force coefficients are of primary interest the time step can be increased moderately
without significant changes. It is also found that the force coefficients alone are not
a good estimators of how the method performs since errors can cancel each other.
This means that a coarser mesh or a longer time step can generate a smaller error
compared to wind tunnel tests but this is not reflecting the actual accuracy of the
method.
Beskrivning
Ämne/nyckelord
Heavy-duty vehicles , Aerodynamics , CFD , Wind tunnel correlation , Absolute accuracy , Temporal resolution